Humidity

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

Humidity

Humidity: the amount of water vapor in the air

The maximum quantity of moisture that can be held in the air depends on air temperature

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

Humidity

Relative Humidity: compares the amount of water vapor present in the air to the maximum amount that the air can hold at that temperature

Expressed as a percentage:

At 100% relative humidity, air is saturated.

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

Humidity

Relative Humidity changes when:

1. Atmosphere gains or loses water vapor

• Evaporation

2. Temperature changes

• Lower temperature relative humidity rises

• Raise temperature relative humidity decreases

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

Humidity

Specific Humidity: actual quantity of water held by a parcel of air

•Grams of water vapor per kilogram of air (g/kg)

•Highest in equatorial zones

•Lowest near poles

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

Dew-point temperature: temperature at which air with a given humidity will reach saturation when cooled without changing its pressure

Humidity

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

The Adiabatic Process

Adiabatic Principle: the physical principle that a gas cools as it expands and warms as it is compressed

Change in temperature:

•caused only by a change in pressure

•not caused by heat flowing in or out of the gas

Visualizing Physical GeographyCopyright © 2008 John Wiley and Sons Publishers Inc.

The Adiabatic Process

As a parcel of air risespressure on the parcel decreasesair expands and cools

As a parcel of air descendspressure on the parcel increasesair is compressed and warms

Atmospheric pressure decreases with altitudeso...